1. Technical Field
This disclosure relates to semiconductor fabrication, and more particularly, to a photomask and method for increasing the aspect ratio of photomask images by employing an assist line.
2. Description of the Related Art
Semiconductor fabrication processes typically include photolithography processes for providing images in a resist material. A resist material is spun onto or otherwise applied to a surface of a layer or layers to be patterned. The resist is exposed to radiation (e.g., light or electrons) to affect the cross-linking properties or to alter the chemistry of the resist in such a way as to render it removable in portions. The portions of the resist are designated as removable by employing a mask for the radiation (e.g., a light or electron mask) which controls the placement of the radiation (e.g., where the radiation falls) on the resist layer.
In high density electronic circuits, it is advantageous not to shrink dimensions of certain elements. For example, contact holes, which shunt a device on a wafer surface with subsequent low resistance metals, are an orifice for current flow. Therefore, a contact with large area is desired. One option to increase the contact size is to draw the contact larger on the photomask. This biasing of the mask compensates for all mechanisms leading to a deviation from the original mask dimensions by providing an offset, which leads to a corrected image. These offsets are normally defined in an iterative way and are strongly dependent on the environment of the features on the photomask, e.g., nested features versus isolated features. In a nested array, the ability to apply biases on the photomask is limited to resolve neighboring features. Another significant limit for applying biases on the photomask is the necessity to resolve and inspect spaces between the features on the actual mask.
Referring to FIG. 1, photo mask features 10 are shown for forming images of deep trenches in a photoresist. In this example, a feature 10 includes a rectangular structure having a length L equal to 2 minimum feature sizes (e.g., 2F) and a width of 1 minimum feature size (e.g., IF). Spacings xe2x80x9csxe2x80x9d, in FIG. 1, are dimensioned as a minimum feature size F. An image projected by the deep trench feature 10 produces a contrast plot as shown in FIG. 2. FIG. 2 is a typical shape of a deep trench image formed by employing feature 10. Iso-contrast line 12 shows the outline of a trench formed in accordance with feature 10 of FIG. 1. Contrast line 12 barely achieves the desired 2:1 aspect ratio of the deep trench. Since spacings between deep trenches are typically one minimum feature size, in order to avoid merging of the features with worst case critical dimension variations, the spacing of features 10 must be carefully monitored to provide an appropriate spacing between two features 10 on the photomask. The spacing accuracy is very tight and includes small spacing tolerances. The inspectable distance between two features or shapes is no smaller than, for example, 120 nm in a one-time image reduction. Consequently, in a 4xc3x97-reticle the minimum distance of two shapes is 480 nm.
Therefore, a need exists for a method and apparatus for increasing the image size and process window for exposing features to be patterned in a resist layer.
A photomask for lithographic processing, in accordance with the present invention, includes a plurality of features for providing an image pattern. The features are arranged in a column on a massk substrate. Each feature is dimensioned to provide an individual image separate from all other images provided by the photomask when exposed to light. A line feature is formed on the mask substrate and extends between and intersects with each of the plurality of features in the column. The line feature extends a length of images produced by the plurality of features arranged in the column when exposed to light wherein the images produced by each of the plurality of features and the line feature remain separate from each other.
A photomask for imaging a trench pattern in a resist layer includes a plurality of trench features arranged in rows and columns on a mask substrate. Each feature is dimensioned to provide an individual image separate from all other images provided by the photomask. Line features are formed on the mask substrate and extend between each of the trench features in each column. The trench features and the line features comprise an image pattern, wherein upon exposure to light of the image pattern, images of the line features extend a column-wise length of images produced by the trench features while the images of each of the trench features remain separate from each other.
In other embodiments, the plurality of features on the mask substrate are preferably spaced apart from neighboring features in the column by greater than a spacing dimension, and the images produced by the plurality of features appear to have the plurality of features separated by the spacing dimension. The line feature may include a width to provide an image of less than a groundrule dimension of a given technology. The plurality of features may include patterns for forming deep trenches or contact holes. The line feature may include a width which is discernable in an image of the photomask adjacent to an intersection point between the features and the line feature, but which is not discernable in the image of the photomask at middle portions of spacings between adjacent features in the column. The plurality of features may be symmetrically or asymmetrically disposed about the line feature. The trench features may include a length, and the images produced by the trench features and the line feature may be such that the trench features appear to be up to 20% longer than the length.
A method for relaxing image placement tolerances for a photomask, in accordance with the present invention, includes the steps of providing a photomask including a plurality of features for providing an image pattern, the features being arranged in a column on a mask substrate, each feature being dimensioned to provide an individual image separate from all other images provided by the photomask, and a line feature for the image pattern, the line feature being formed on the mask substrate and extending between each of the plurality of features in the column, the line feature for extending images produced by the plurality of features arranged in the column wherein the images of each of the plurality of features remain separate from each other, and directing light through the photomask onto a resist layer to expose the resist layer in accordance with a pattern provided by the photomask, such that images of the plurality of features are extended in the direction of the column due to the line feature.
In other methods, the step of providing may include the step of spacing the plurality of features on the substrate from neighboring features in the column by greater than a spacing dimension wherein the images produced by the plurality of features appear to be separated by the spacing dimension. The line feature may include a width less than a groundrule dimension of a given technology. The plurality of features may include patterns for forming one of deep trenches and contacts holes. The step of providing may include the step of providing a width of the line feature which is discernable in an image of the photomask adjacent to an intersection point between the features and the feature line, but which is not discernable in the image of the photomask at middle portions of spacings between adjacent features in the column. The step of providing may include the step of providing the plurality of features symmetrically or asymmetrically about the line feature. The method may further include the step of adjusting a width of the line feature to alter a length of the plurality of features.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.